The present invention relates generally to a new and novel quenching apparatus and method for hardening steel parts. More particularly, the present invention relates to a quenching apparatus and method for hardening steel parts which achieves high strength and surface compressive stresses on all steels, including relatively low-alloy and standard carbon steels, and water and water based solutions as the quenching agent.
The quenching apparatus and method for hardening steel parts in accordance with the present invention relates generally to the heat treatment of steel parts, including carburized steel parts, steel parts heated by induction heating and other steel parts heated in electric, atmosphere, gas and vacuum furnaces. The invention has application in the metallurgic industry, including heat treating, machine construction, bearing and tool production, as well as in other branches of industry.
A steel quenching method where the depth of the quenched surface layer is controlled, which increases service life, is described in xe2x80x9cNew Induction Hardening Technology,xe2x80x9d authored by K. Z. Shepelyakovskii and F. V. Bezmenov which appeared on pages 225 through 227 of the October 1998, publication xe2x80x9cAdvanced Materials and Processes.xe2x80x9d Steel quenched using this method generally has low depth of hardened layer and fine grain with arrested growth of austenite grains at high temperatures. Due to limited hardenability, compressive stresses appear on the surface of such steel parts and the fine grain provides high strength. In addition to providing an increase in the service life of such heat treated steel parts, there is an opportunity to replace relatively expensive high-alloy steels with less expensive low-alloy steels and replace fire and environmentally dangerous quench oils with water and water based quenching solutions. However, the depth of hardness in steel parts hardened using this method is controlled by the chemical composition of the steel parts being hardened.
Steel quenching where the depth of the hardened surface layer is controlled in accordance with this method is made in water jets. The service life of such heat treated steel parts where the depth of the hardened surface layer is controlled generally increases when compared to oil quenching. However, it is often necessary to select or create an appropriate alloy of steel for use in steel parts having different configurations and sizes to obtain the effect of high surface compressive stresses.
In addition, with this quenching method no criteria exists to calculate the rate of water flow for steel parts having different configurations and/or sizes. Thus, a relatively high water flow rate is normally chosen for all steel parts which is not always justified and results in unnecessary energy expenses and makes the industrial process more complicated than necessary. While the high service life of steel parts where the depth of the hardened surface layer is controlled is considered an advantage for certain steel grades, other steel grades can also achieve the effect of increased strength (as compared to known prior art steel part quenching methods) and high residual compressive surface stresses if the heat treating parameters are properly controlled. In this method of heat treating steel, induction heating is primarily used and, to the applicant""s knowledge, there is no data regarding oven heating, including such data for carburized parts, and the industrial regimes are not optimized. Thus, the heat treating method described above is entirely dependent on the composition of the steel alloys available. As a practical matter, it may be difficult to obtain steel alloys having a suitable composition. Accordingly, in practice, the hardening method should be adapted to those steel alloys which are available.
Another known prior art steel quenching method is described in xe2x80x9cIntense Quenchingxe2x80x9d authored by Roy F. Kern and published on pages 19 through 23 in the No. 9 issue of xe2x80x9cHeat Treatingxe2x80x9d in 1986. This known prior art steel quenching method involves xe2x80x9cshell hardening,xe2x80x9d which results in uniform quenching of all of the surface to a certain depth until reaching high hardness using intensive jet cooling. In this method, the examples of the application of medium-carbon 1045 steel are given. One advantage of this method is the opportunity to increase the service life of steel parts using standard carbon steels, rather than alloy steels where the depth of the hardened surface layer is controlled by the composition of the steel. However, this method also has many of the disadvantages present in the previous method described. Namely, as discussed in prior publications authored by the applicant, no consideration is given to the parameters necessary to optimize the depth of the hardened surface layer, and the following correlation that the depth of the hardened surface layer should be changed for steel parts having different configurations and/or sizes is ignored:             Δ      ⁢              xe2x80x83            ⁢      δ        D    =  constant
where:
xcex94xcex4 is the optimum hardened depth; and
D is the cross-sectional thickness.
This correlation was developed by the applicant and is considered to provide a foundation for the quenching apparatus and method for hardening steel parts in accordance with the present invention.
In addition, this method does not have any criteria allowing the calculation of the optimum cooling solution quench flow and the technological process is not optimized.
Another steel quenching method is described in Japanese patent application number 61-48514 to Naito Takeshi, published Aug. 16, 1984, for a xe2x80x9cMethod of Steel Quenchingxe2x80x9d now Japanese Patent No. 59-170039. In this method, alloy steel parts are quenched in such a manner that a hard surface layer of a given depth and an arbitrarily hard matrix are obtained. For given steel grades, ranges for hardening regimes are found by experimentation to increase the service life of such steel parts. One example of this method involves an alloy steel specimen containing 0.65% to 0.85% carbon, 0.23% to 0.32% silicon, 0.4% to 0.9% manganese, approximately 2% nickel, 0.5% to 1.5% chromium and 0.1% to 0.2% molybdenum which is heated to 800xc2x0 C. to 850xc2x0 C. and spray quenched with water fed under a 0.4 to 0.6 MPa pressure for 0.2 to 0.8 seconds. The steel specimen is then isothermally heated at 150xc2x0 C. to 250xc2x0 C. for ten (10) to fifty (50) minutes. One disadvantage of this method is that it considers only high-carbon alloy steels. Also, the depth of the hard surface layer is not optimal for steel parts having different configurations and/or sizes and, because of this, steel strengthening is not consistently achieved in all parts. In addition, this method does not taken into consideration the optimization of the quenchant solution circulation rate.
A steel quenching method described in Ukraine Patent No. UA 4448, Bulletin No. 6-1, to N. I. Kobasko, in 1994, describes heating, cooling until the appearance of maximum compressive surface stresses, followed by isothermal heating (tempering). This method is based on cooling in the range of 0.8xe2x89xa6Knxe2x89xa61, where Kn is the Kondratjev number, until reaching maximum compressive surface stresses, then isothermally heating at martensite start temperature Ms until the complete transformation of the overcooled austenite of the matrix occurs and tempering. The Kondratjev number characterizes the intensity of cooling and is variable between zero (0) and one (1). It is the ratio between usual cooling and cooling when heat transfer is infinite. Therefore, even during very intense cooling, this ratio cannot exceed one (1).
One disadvantage of this method is that it deals only with alloy steels. To reach the maximum compressive surface stresses on the surface the cooling is stopped and due to this interruption in cooling, the effect of greater than normal steel strength is not fully achieved. In addition, there is no method to calculate the optimal rate of quenchant solution flow to ensure that increased strength (as compared to known prior art steel part quenching methods) is consistently realized.
Thus, in summary, an analysis of known prior art methods of steel quenching shows that steel quenching with the formation of a hard surface layer of a given depth has greater advantages than through quenching. However, a common disadvantage of these known prior art steel quenching methods is that there is no change in the optimum depth of the hard surface layer for steel parts having different configurations and/or sizes. In addition, in known prior art methods of steel quenching, the quenchant solution circulation rate is not optimized to preclude the development of xe2x80x9cself-regulated thermal processxe2x80x9d (when there is nucleate boiling heat transfer on the steel part surface and the steel part surface temperature is changing very slowly and is nearly constant and is close to the quenchant boiling temperature). Therefore, the steel part strength cannot be greater than the normal steel part strength. In order to provide additional steel part strengthening, the xe2x80x9cself-regulated thermal processxe2x80x9d should be avoided.
Accordingly, an object of the present invention is to provide a quenching apparatus and method for heat treating steel parts where the effect of increased strength (as compared to known prior art steel part quenching methods) and high compressive surface stresses are achieved for all alloy grades and standard carbon grades of steel, and lower distortion and cracking resulting from the quenching process.
Another object of the present invention is the provision of a quenching apparatus and method for heat treating steel parts which utilizes water or a water based quenchant solution rather than expensive, flammable and environmentally dangerous oil based quenchant materials.
A preferred embodiment of the present invention is, therefore, directed to a quenching apparatus and method for hardening a multitude of alloy steel parts used in, for example, metallurgy, machine construction, bearing and tool industry, quenching of carburized parts; parts heated by induction, salt bath, the usual oven heating and vacuum furnaces. Optimal cooling conditions prevent the xe2x80x9cself-regulated thermal processxe2x80x9d from occurring while optimizing the depth of the hardened layer and providing increased strength (as compared to known prior art steel part quenching methods) for the entire steel part. The optimum depth of hardening is considered to be when surface compressive stresses are at their maximum value and depth. In addition, xe2x80x9cmaximumxe2x80x9d steel strength is achieved when the cooling rate is above a certain minimum level. However, additional strengthening can occur when the rate of cooling avoids both film boiling and subsequent xe2x80x9cself regulated thermal process,xe2x80x9d and goes directly to convection cooling. This is subsequently referred to as xe2x80x9cdirect convection cooling.xe2x80x9d xe2x80x9cDirect convection coolingxe2x80x9d is present in the quenching operation when the Biot number is between five (5) and fifty (50). xe2x80x9cDirect convection coolingxe2x80x9d is maintained when there is sufficient coolant movement at the surface of the part being quenched to eliminate shock boiling, film boiling and nucleate boiling everywhere on the part""s surface. At the end of the quench, some steel parts will benefit from isothermal cooling in the air (self tempering).
This process method results in additional strengthening of steel parts and maximum compressive surface stresses are achieved, resulting in increased service life of the steel parts. Relatively expensive alloy and high-alloy steels can be replaced with less expensive low-alloy or standard carbon steels. In the alternative, when using low-alloy grades of steel, such as 1010 or 1541 grades of steel, or alloy-carburized grades of steel, such as 9610 or 8620 grades of steel, the carburization cycle can be significantly reduced or eliminated entirely by hardening the steel parts using xe2x80x9cdirect convection cooling.xe2x80x9d This is because the severity of the xe2x80x9cdirect convection coolingxe2x80x9d quench drives the hardness deeper into the surface of the steel parts and creates higher compressive stresses. Also, instead of an oil based quenching material, water or a water based solution is used. Thus, the quenching apparatus and method for hardening steel parts improves the ecological state of the environment and increases labor efficiency in hardening steel parts.
In particular, the present invention is directed to a quenching apparatus and method for hardening steel parts which includes heating the steel parts, xe2x80x9cdirect convection coolingxe2x80x9d the steel parts until the appearance of maximum compressive stresses on the surface of the steel parts, and then self-tempering or tempering.
The required heat transfer for xe2x80x9cdirect convection coolingxe2x80x9d on the surface of the steel parts being hardened is determined from the following formula:   Bi  ≥            2      ⁢              (                              ϑ            0                    -                      ϑ            1                          )                            ϑ        Hed            +              ϑ        1            
where   Bi  =      hR    λ  
xe2x80x83The Biot number, a dimensionless value;
r=The radius of the steel part;
h=The heat transfer coefficient;
xcex=The thermal conductivity;
D=2R The characteristic size of the part (diameter, thickness of plate, etc.; R is the radius);
xcex80=T0-TK;
T0=The austenization temperature;
TK=The quenchant boiling temperature;
xcex81=The superfluous temperature at the beginning of xe2x80x9cthe self-regulated thermal processxe2x80x9d (nucleate boiling);
xcex8Hed=TK-TC; and
TC=The temperature of the quenching bath.
When using xe2x80x9cdirect convection cooling,xe2x80x9d the optimum depth for the steel parts will be in the range of one percent (1%) of the part cross-sectional thickness to all the way through the part depending on the composition of the steel and the configuration and size of the steel parts being hardened. The time to interrupt the quench when surface compressive stresses in the steel parts being hardened are at their maximum is calculated using the formula:   τ  =            K              a        ⁢                  xe2x80x83                ⁢        Kn              ⁢          (              b        +                  0.24          ⁢          k                    )      
where
K=The Kondratjev form factor;
Kn=The Kondratjev number (0.6xe2x89xa6Knxe2x89xa71);
a=The thermal diffusivity;
b=A parameter dependent on the austenizing temperature of the steel parts being hardened and the cooling medium temperature;
k=1, 2 or 3 for plate-shaped, cylinder-shaped or ball-shaped bodies, respectively;       b    =                  I        n            ⁢                                    T            0                    -                      T            C                                                T            Core                    -                      T            C                                ;
xe2x80x83and
Tcore=The core temperature
The presence of compressive surface stresses from using the present method is in contrast to the neutral or tensile stresses found in parts using traditional methods, such as oil and polymer/water quenchants, where the Biot number is less than five (5).
To use this formula, in most cases the xe2x80x9ccorexe2x80x9d temperature can be estimated in the range of 400xc2x0 C. to 450xc2x0 C. (This temperature can be further quantified, if desired, by experimentation.) If the timing of interrupting the quench of the steel parts being hardened is off, the level of surface compressive stresses in the steel parts being hardened may be less than the potential xe2x80x9cmaximumxe2x80x9d level of surface compressive stresses calculated and the maximum possible for the steel parts being hardened.
Other advantages and novel features of the present invention will become apparent in the following detailed description of the invention when considered in conjunction with the accompanying drawings.